This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/JP99/00757 which has an International filing date of Feb. 22, 1999, which designated the United States of America.
The present invention relates to a polyoxymethylene resin composition and the molded product thereof, which exhibits excellent resistance to friction and wearing, high stiffness, high repeated impact strength, and small molding strain, and is excellent in gear precision.
A polyoxymethylene resin has been widely used as an engineering resin having well-balanced mechanical properties and excellent moldability in fields such as automobile parts, electrical and electronic products, etc. In the field of electrical and electronic products, downsizing and lightening of products have been recently proceeding. As a result, the polyoxymethylene resin is required, not only for further improvement in stiffness and resistance to friction and wearing, but also for improvement in impact strength, molding strain and gear precision.
For the purpose of improving stiffness and resistance to friction and wearing, techniques of incorporating an inorganic filler, a lubricant and the like with a polyoxymethylene resin have been proposed. Japanese Patent Publication Unexamined No. 1-263145 (corresponding to U.S. Pat. No. 5,106,896) discloses a technique of adding inorganic powder having a mean particle diameter of 100 xcexcm or less and an ester of mono- or poly-valent alcohol and fatty acid to a polyacetal resin. Japanese Patent Publication Unexamined No. 3-111446 (corresponding to U.S. Pat. No. 5,173,532) discloses a resin composition obtainable by incorporating with a polyacetal resin a graft copolymer wherein an olefin polymer and a vinyl or ether polymer are chemically bonded in a branch or cross-linking structure, a lubricant, and inorganic powder having a mean particle diameter of 100 xcexcm or less. These prior art documents do not exemplify, as the inorganic filler, wollastonite which is one of the components of the present invention; therefore, they are totally insufficient in improvement of stiffness and resistance to friction and wearing. Japanese Patent Publication Unexamined No. 5-51514 discloses a technique of adding spindle-shaped calcium carbonate and an ester of mono- or poly-valent alcohol and fatty acid to a polyacetal resin, but this is also insufficient in improving stiffness and resistance to friction and wearing. Japanese Patent Publication Unexamined No. 7-157630 discloses a composition comprising a graft or block copolymer obtained from olefin polymers and at least one of vinyl polymers, fatty ester, inorganic powder having a mean particle diameter of 30 xcexcm or less and potassium titanate whisker, and a polyacetal resin. The composition disclosed therein is sufficient in improving stiffness, but is still insufficient in improving resistance to friction and wearing. Further, it has a drawback that molding strain (warpage of a molded product) is large.
On the other hand, techniques using wollastonite for a polyoxymethylene resin are described in Japanese Patent Publication Unexamined No. 49-21451 (corresponding to U.S. Pat. No. 3,775,363) which discloses an intimate mixture comprising 98 to 25 parts by weight of a polyoxymethylene resin and 2 to 75 parts by weight of needle-like calcium metasilicate without using a coupling agent. This composition is improved in processability, thermal stability and dimensional accuracy compared with the case using a glass fiber. Japanese Patent Publication Unexamined No. 61-120848 (corresponding to U.S. Pat. No. 4,645,785) discloses a composition having high resistance to wearing comprising 40 to 94.7 parts by weight of a polyoxymethylene, 5 to 50 parts by weight of wollastonite, 0.2 to 5 parts by weight of N-hydroxymethylmelamine, 0.1 to 5 parts by weight of silicone oil having a polymerization degree of 10 to 5,000, 0.1 to 10 parts by weight of polyethylene or a copolymer of ethylene and xcex1-olefin having a molecular weight of 100,000 to 1,000,000. It is observed that this composition is improved in resistance to friction and wearing with metals, but the resistance to friction and wearing with a polyoxymethylene resin or the same composition, which is to be necessary for practical use, is considerably insufficient. Japanese Patent Publication Unexamined No. 64-43555 (corresponding to U.S. Pat. No. 4,987,176) discloses a composition containing polyoxymethylene polymers, wollastonite having a mean thickness of 5 xcexcm or less and a mean length/thickness ratio of 5 or more, and if necessary additives, and a production method thereof. Further, this prior art specifically discloses, as additives, melamines such as N-hydroxymethylmelamine, silicone oil having a polymerization degree n of 10 to 5,000 and a polyethylene and/or a copolymer of ethylene and xcex1-olefin having a molecular weight of 100,000 to 1,000,000, and this prior art further discloses as more preferable additives, stabilizers, nucleating agents, antistatic agents, light stabilizers, flame-retardants, lubricants, plasticizers, pigments, dyes, thermal stabilizers, and mold release agents. However, this prior art does not teach specific examples of lubricants. The composition disclosed in this prior art purposes improving mechanical strength.
The present inventors have made studies about adding various inorganic fillers, lubricants and other resin components to a polyoxymethylene resin. As a result, they accomplished the present invention by finding a composition which comprises (A) a polyoxymethylene resin, (B) wollastonite, (C) at least one member selected from the group consisting of an ester of (i) an alcohol and a fatty acid, (ii) an ester of an alcohol and a dicarboxylic acid, and (iii) a compound of a polyoxyalkylene glycol, and (D) a polyolefin resin, whereby said composition exhibits excellent resistance to friction and wearing, high stiffness, high repeated impact strength, and small molding strain, and is excellent in gear precision.
Accordingly, the present invention relates to a polyoxymethylene resin composition comprising:
(A) 50 to 99 parts by weight of a polyoxymethylene resin;
(B) 1 to 50 parts by weight of a wollastonite having a volume-average particle diameter of 0.5 to 40 xcexcm;
(C) 0.1 to 10 parts by weight of at least one member selected from the group consisting of (i) an ester of an alcohol and a fatty acid, (ii) an ester of an alcohol and a dicarboxylic acid, and (iii) a compound of a polyoxyalkylene glycol based on 100 parts by weight of the total amount of Component (A) and Component (B); and
(D) 0.1 to 10 parts by weight of a polyolefin resin based on 100 parts by weight of the total amount of Component (A) and Component (B).
Further, the present invention relates to a molded product prepared by molding the above composition, particularly working parts such as a gear, a cam, a slider, a lever, an arm, a clutch, a joint, an axis, a bearing, a key-stem and a key-top, outsert resinous parts for a chassis, a chassis, a tray and a side plate, which are used for office automation apparatuses represented by a printer and a copying machine; for video apparatuses represented by a video tape recorder (VTR) and a video movie (a video camera); for apparatuses for music, image, or information represented by a cassette player, a laser disc (LD), a mini disc (MD), a compact disc (CD) [including CD-ROM (read only memory), CD-R (recordable) and CD-RW (rewritable)], a digital video disc (DVD) [including DVD-ROM, DVD-R, DVD-RAM (random access memory) and DVD-Audio], a navigation system and a mobile personal computer; for telecommunication apparatuses represented by a cellular phone and a facsimile machine; for an interior or exterior working part for an automobile; and for industrial miscellaneous goods represented by a disposable camera, a toy, a fastener, a conveyor, a buckle, and an apparatus for house-building.
The polyoxymethylene resins used for Component (A) of the present invention are homopolymers prepared by polymerizing formaldehyde or a cyclic oligomer thereof such as trioxane, a trimer of formaldehyde, and tetraoxane, a tetramer of formaldehyde, and by blocking terminals of the polymer with an ether group or an ester group; copolymers prepared by copolymerizing formaldehyde, its trimer of trioxane or its tetramer of tetraoxane with comonomers such as ethylene oxide, propylene oxide, 1,3-dioxolan and 1,4-butanediol formal; copolymers thereof which further have branched molecular chains; polyoxymethylene block copolymers, of which one terminal is blocked with different type of components such as polyethylene glycol; and the like.
The polyoxymethylene resins used in the present invention has a melt index (MI) (measured under the conditions prescribed by ASTM-D 1238-57T) of 0.1 g/10 min to 150 g/10 min, preferably 1 g/10 min to 100 g/10 min. The copolymer type polyoxymethylene contains the comonomers in an amount of 0.1 to 20 mol, preferably 0.3 to 10 mol, based on 100 mol of oxymethylene.
The polyoxymethylene resin of the present invention can contain a heat stabilizer, a weathering (light) stabilizer or a combination of these stabilizers for use. Of course, the polyoxymethylene resin can be preliminarily stabilized with these stabilizers.
As the heat stabilizer, an antioxidant and a catching agent of formaldehyde or formic acid are preferable. A combination of these heat stabilizers is also effective. As the antioxidants, a hindered phenol type antioxidant is preferable. For example, the hindered phenol type antioxidant includes n-octadecyl-3-(3xe2x80x2,5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxyphenyl)-propionate, n-octadecyl-3-(3xe2x80x2-methyl-5xe2x80x2-t-butyl-4xe2x80x2-hydroxyphenyl)-propionate, n-tetradecyl-3-(3xe2x80x2,5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxyphdnyl)-propionate, 1,6-hexanediol-bis-(3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate), 1,4-butanediol-bis-(3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate), triethylene glycol-bis-(3-(3-t-butyl-S-methyl-4-hydroxyphenyl)-propionate), tetrakis-(methylene-3-(3xe2x80x2,5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxyphenyl)propionatemethane, 3,9-bis(2-(3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy)-1,1-dimethylethyl)2,4,8,10-tetraoxaspiro(5,5)undecane, N,Nxe2x80x2-bis-3-(3xe2x80x2,5xe2x80x2-di-t-butyl-4-hydroxyphenol)propionylhexamethylenediamine, N,Nxe2x80x2-tetramethylenebis-3-(3xe2x80x2-methyl-5xe2x80x2-t-butyl-4-hydroxyphenol)propionyldiamine, N,Nxe2x80x2-bis-(3-(3,5-di-t-butyl-4-hydroxyphenol)propionyl)hydrazine, N-salicyloyl-Nxe2x80x2-salicylidene hydrazine, 3-(N-salicyloyl)amino-1,2,4-triazol, N,Nxe2x80x2-bis(2-(3-(3,5-di-butyl-4-hydroxyphenyl)propionyloxy)ethyl)oxyamido, and the like. Of these hindered phenol type antioxidants, triethylene glycol-bis-(3-(3-t-butyl-5-methyl-4-hydroxyphenyl)-propionate), tetrakis-(methylene-3-(3xe2x80x2,5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxyphenyl)propionatemethane are preferable.
As the catching agents of formaldehyde and formic acid, there can be exemplified (a) compounds and polymers containing formaldehyde-reactive nitrogen, (b) a hydroxide, an inorganic acid salt, a carboxylic acid salt or an alkoxide, and the like of an alkali metal or an alkali earth metal.
As (a) the compounds containing formaldehyde-reactive nitrogen, there can be exemplified (1) dicyan diamide, (2) an amino-substituted triazine, (3) a co-condensation product of an amino-substituted triazine and formaldehyde, and the like. (2) an amino-substituted triazine includes, for instance, guanamine(2,4-diamino-sym-triazine), melamine(2,4,6-triamino-sym-triazine), N-butylmelamine, N-phenylmelamine, N,N-diphenylmelamine, N,N-diallylmelamine, N,Nxe2x80x2,Nxe2x80x3-triphenylmelamine, N-methylolmelamine, N,Nxe2x80x2-dimethylolmelamine, N,Nxe2x80x2,Nxe2x80x3-trimethylolmelamine, benzoguanamine(2,4-diamino-6-phenyl-sym-triazine), 2,4-diamino-6-methyl-sym-triazine, 2,4-diamino-6-butyl-sym-triazine, 2,4-diamino-6-benzyloxy-sym-triazine, 2,4-diamino-6-butoxy-sym-triazine, 2,4-diamino-6-cyclohexyl-sym-triazine, 2,4-diamino-6-chloro-sym-triazine, 2,4-diamino-6-mercapto-sym-triazine, 2,4-dioxy-6-amino-sym-triazine, 2-oxy-4,6-diamino-sym-triazine, N,Nxe2x80x2,Nxe2x80x2-tetracyanoethylbenzoguanamine, and the like. (3) a co-condensation product of an amino-substituted triazine and formaldehyde includes, for instance, a melamine-formaldehyde polycondensation product, and the like. Of these, dicyan diamide, melamine and a melamine-formaldehyde polycondensation product are preferable.
Further, as (a) the polymer containing a formaldehyde-reactive nitrogen group, there can be exemplified (1) a polyamide resin, (2) a polymer obtained by polymerizing acrylamide and/or a derivative thereof, or acrylamide and/or a derivative thereof and other vinyl monomers in the presence of metal alcolate, (3) a polymer obtained by polymerizing acrylamide and/or a derivative thereof, or acrylamide and/or a derivative thereof and other vinyl monomers in the presence of a radical polymerization initiator, and (4) a polymer containing nitrogen-containing groups such as an amine, an amide, urea and urethane. As (1) a polyamide resin, there can be exemplified nylon 4-6, nylon 6, nylon 6-6, nylon 6-10, nylon 6-12, nylon 12 and copolymers thereof such as nylon 6/6-6, nylon 6/6-6/6-10 and nylon 6/6-12. As (2) a polymer obtained by polymerizing acrylamide and/or a derivative thereof, or acrylamide and/or a derivative thereof and other vinyl monomers in the presence of metal alcolate, a poly-xcex2-alanine copolymer can be exemplified. These polymers can be prepared according to the methods disclosed in Japanese Patent Publication Examined No. 6-10259 (corresponding to U.S. Pat. No. 5,015,707), Japanese Patent Publication Examined No. 5-87096, Japanese Patent Publication Examined No. 5-47568 and Japanese Patent Publication Unexamined No. 3-234729. (3) a polymer obtained by polymerizing acrylamide and/or a derivative thereof, or acrylamide and/or a derivative thereof and other vinyl monomers in the presence of a radical polymerization initiator can be prepared according to the method disclosed in Japanese Patent Publication Unexamined No. 3-28260 (corresponding to U.S. Pat. No. 5,011,890).
(b) The hydroxide, the inorganic acid salt, the carboxylic acid salt or the alkoxide of an alkali metal or an alkali earth metal includes, for instance, hydroxide of such as sodium, potassium, magnesium, calcium and barium, and carbonate, phosphate, silicate, borate and carboxylate of the above metals. The carboxylic acid of the carboxylate is saturated or unsaturated aliphatic carboxylic acids having 10 to 36 carbon atoms, and the like. And, these carboxylic acids may be substituted with hydroxyl groups. As the saturated aliphatic carboxylic acids, a capric acid, a lauric acid, a myristic acid, a palmitic acid, a stearic acid, an arachic acid, a behenic acid, a lignoceric acid, a cerotic acid, a montanoic acid, a melissic acid and a lacceric acid can be exemplified. As the unsaturated aliphatic carboxylic acids, an undecylenic acid, an oleic acid, an elaidic acid, cetoleic acid, an erucic, acid, a brassidic acid, a sorbic acid, a linoleic acid, a linolenic acid, an arachidonic acid, a propiolic acid, a stearolic acid and the like can be exemplified. Further, as an alkoxide, a methoxide, an ethoxide and the like of the above-exemplified metals can be illustrated as examples.
As the weathering (light) stabilizers, (a) benzotriazole type substances, (b) anilide oxalate type substances and (c) hindered amine type substances are preferable. As (a) the benzotriazole type substances, there can be exemplified, for instance, 2-(2xe2x80x2-hydroxy-5xe2x80x2-methyl-phenyl)benzotriazole, 2-(2xe2x80x2-hydroxy-3,5-di-t-butyl-phenyl)benzotriazole, 2-(2xe2x80x2-hydroxy-3,5-di-t-amyl-phenyl)benzotriazole, 2-[2xe2x80x2-hydroxy-3,5-bis-(xcex1,xcex1-dimethylbenzyl)phenyl]-2H-benzotriazole, 2-(2xe2x80x2-hydroxy-5xe2x80x2-octylphenyl)benzotriazole, and the like. Of these, 2-[2xe2x80x2-hydroxy-3,5-bis-(xcex1,xcex1-dimethylbenzyl)phenyl]-2H-benzotriazole and 2-(2xe2x80x2-hydroxy-3,5-di-t-butyl-phenyl)benzotriazole are preferable.
As (b) the anilide oxalate type substances, there can be exemplified, for instance, 2-ethoxy-2xe2x80x2-ethyloxalic acid bisanilide, 2-ethoxy-5-t-butyl-2xe2x80x2-ethyloxalic acid bisanilide, 2-ethoxy-3xe2x80x2-dodecyloxalic acid bisanilide, and the like. These substances may be used alone or in combination.
As (c) the hindered amine type substances, there can be exemplified 4-acetoxy-2,2,6,6-tetramethylpiperidine, 4-stearoyloxy-2,2,6,6-tetramethylpiperidine, 4-acryloyloxy-2,2,6,6-tetramethylpiperidine, 4-(phenylacetoxy)-2,2,6,6-tetramethylpiperidine, 4-benzoyloxy-2,2,6,6-tetramethylpiperidine, 4-methoxy-2,2,6,6-tetramethylpiperidine, 4-stearyloxy-2,2,6,6-tetramethylpiperidine, 4-cyclohexyloxy-2,2,6,6-tetramethylpiperidine, 4-benzyloxy-2,2,6,6-tetramethylpiperidine, 4-phenoxy-2,2,6,6-tetramethylpiperidine, 4-(ethylcarbamoyloxy)-2,2,6,6-tetramethylpiperidine, 4-(cyclohexylcarbamoyloxy)-2,2,6,6-tetramethylpiperidine, 4-(phenylcarbamoyloxy)-2,2,6,6-tetramethylpiperidine, bis(2,2,6,6-teteramethyl-4-piperidine)-carbonate, bis(2,2,6,6-tetramethyl-4-piperidyl)-oxalate, bis(2,2,6,6-tetramethyl-4-piperidyl)-malonate, bis(2,2,6,6-teteramethyl-4-piperidyl)-sebacate, bis(2,2,6,6-teteramethyl-4-piperidyl)-adipate, bis(2,2,6,6-teteramethyl-4-piperidyl)-terephthalate, 1,2-bis(2,2,6,6-teteramethyl-4-piperidyloxy)-ethane, xcex1-xcex1xe2x80x2-bis(2,2,6,6-teteramethyl-4-piperidyloxy)-p-xylene, bis(2,2,6,6-teteramethyl-4-piperidyl)tolylene-2,4-dicarbamate, bis(2,2,6,6-teteramethyl-4-piperidyl)-hexamethylene-1,6-dicarbamate, tris(2,2,6,6-tetramethyl-4-piperidyl)-benzene-1,3,5-tricarboxylate, tris(2,2,6,6-tetramethyl-4-piperidyl)-benzene-1,3,4-tricarboxylate and the like, preferably bis(2,2,6,6-tetramethyl-4-piperidyl)-sebacate. These hindered amine type substances may be used individually or in combination. Further, the combination of at least one of the benzotriazole type substances and the anilide oxalate type substances with the hindered amine type substances is most preferable.
The wollastonite used for Component (B) of the present invention is prepared by generally pulverizing wollastonite existing in the nature or synthetic wollastonite. In this pulverizing process, wollastonite in the shape of a long thin needle and a particle (including a short stick and a granule) is obtained. These wollastonites may be in the shape of a needle or a particle, or in combination of these shapes. The particle diameter is in the range of 0.5 to 40 xcexcm, preferably 1 to 30 xcexcm, in a volume-average particle diameter. The particle diameter of over 40 xcexcm unfavorably causes deterioration of surface appearance and sliding property of the resin. The particle diameter of less than 0.5 xcexcm unfavorably causes deterioration of processability since the improvement effect of stiffness is reduced and the viscosity at melting is increased.
The granular wollastonite has an aspect ratio of preferably 2 to 7, more preferably 3 to 5. When the aspect ratio is lower than 2, the stiffness is not improved effectively. When it is higher than 7, the warpage is apt to become large. The needle-like wollastonite has an aspect ratio of preferably 10 to 30, more preferably 10 to 25. When an aspect ratio is lower than 10, the stiffness is not improved effectively. When it is higher than 30, the warpage is apt to become large.
Either wollastonite with a treated surface or with an untreated surface may be used. As the surface treatment agents, conventionally known ones can be used; for instance, various types of coupling agents such as silane type, titanate type, aluminum type and zirconium type can be used. Specifically, the useful coupling agents include N-(2-aminoethyl)-3-aminopropyl triethoxysilane, 3-glycidoxypropyl trimethoxysilane, isopropyl trisstealoyl titanate, diisopropoxy ammonium ethylacetate, n-butylzirconate, and the like.
The wollastonite is added in an amount of 1 to 50 parts by weight, preferably 2 to 40 parts by weight, more preferably 5 to 30 parts by weight, based on 100 parts by weight of the resin comprising the polyoxymethylene type resin and the wollastonite. When it is less than 1 part by weight, the reinforcing effect of the filler is not exhibited sufficiently. When it is more than 50 parts by weight, not only the deterioration in the surface appearance but also reduction in molding processability and impact strength are unfavorably caused.
The ester of an alcohol and a fatty acid, the ester of an alcohol and a dicarboxylic acid and the polyoxyalkylene glycol compound used as Component (C) of the present invention are as follows.
The ester of an alcohol and a fatty acid includes esters of an alcohol and a fatty acid listed below.
The alcohol includes a monovalent alcohol and a polyvalent alcohol. The monovalent alcohol includes, for instance, saturated or unsaturated alcohol such as methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, amyl alcohol, hexyl alcohol, heptyl alcohol, octyl alcohol, nonyl alcohol, decyl alcohol, undecyl alcohol, lauryl alcohol, tridecyl alcohol, myristyl alcohol, pentadecyl alcohol, cetyl alcohol, heptadecyl alcohol, stearyl alcohol, oleyl alcohol, nonadecyl alcohol, eicosyl alcohol, ceryl alcohol, behenyl alcohol, melissyl alcohol, hexyldecyl alcohol, octyldodecyl alcohol, decylmyristyl alcohol, and decylstearyl alcohol.
The polyvalent alcohol includes, for instance, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, butane diol, pentane diol, hexane diol, glycerin, diglycerin, triglycerin, pentaerythritol, arabitol, ribitol, xylitol, sorbite, sorbitan, sorbitol, mannitol, and the like.
The fatty acid includes caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, undecyl acid, lauric acid, tridecyl acid, myristic acid, pentadecyl acid, palmitic acid, pentadecyl acid, stearic acid, nanodecanoic acid, arachic acid, behenic acid, lignoceric acid, cerotic acid, heptacosanoic acid, montanoic acid, melissic acid, lacceric acid, undecylenic acid, oleic acid, elaidic acid, cetoleic acid, erucic acid, brassidic acid, sorbic acid, linoleic acid, linolenic acid, arachidonic acid, propiolic acid, stearolic acid, and the like. Further, the fatty acid includes natural fatty acids containing the above-listed components or a mixture thereof. These fatty acids may be substituted with a hydroxy group. Among the above-listed esters of an alcohol and a fatty acid, an ester of an alcohol and a fatty acid independently having a carbon number of 10 or more is preferable from the viewpoint of improvement in resistance to friction and wearing; an ester of a fatty acid having a carbon number of 12 or more and an alcohol having a carbon number of 10 or more is more preferable; and an ester of a fatty acid having a carbon number of 12 to 30 and an alcohol having a carbon number of 10 to 20 is more preferable.
The ester of an alcohol and a dicarboxylic acid includes the monoester and diester of a saturated or unsaturated primary alcohol such as octyl alcohol, nonyl alcohol, decyl alcohol, undecyl alcohol, lauryl alcohol, tridecyl alcohol, myristyl alcohol, pentadecyl alcohol, cetyl alcohol, heptadecyl alcohol, stearyl alcohol, oleyl alcohol, nonadecyl alcohol, eicosyl alcohol, ceryl alcohol, behenyl alcohol, melissyl alcohol, hexyldecyl alcohol, octyldodecyl alcohol, decylmyristyl alcohol and decylstearyl alcohol and a dicarboxylic acid such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, maleic acid and fumaric acid, and a mixture thereof.
Of these esters of an alcohol and a dicarboxylic acid, an acid of an alcohol and a dicarboxylic acid having independently a carbon number of 10 or more is preferable.
As the polyoxymethylene glycol compound, three types of the compound are exemplified. A first group of the compound includes a polycondensation product containing an alkylene glycol as a monomer, for example, a polyethylene glycol, a polypropylene glycol and a block polymer of ethylene glycol and propylene glycol. The polymerization degree of these compounds is preferably 5 to 1,000, more preferably 10 to 500. A second group is an etherified compound of the first group of compounds and an aliphatic alcohol, for example, polyethylene glycol oleyl ether (ethyleneoxide polymerization degree: 5 to 50), polyethylene glycol cetyl ether (ethyleneoxide polymerization degree: 5 to 50), polyethylene glycol stearyl ether (ethyleneoxide polymerization degree: 5 to 30), polyethylene glycol lauryl ether (ethyleneoxide polymerization degree: 5 to 30), polyethylene glycol tridecyl ether (ethyleneoxide polymerization degree: 5 to 30), polyethylene glycol nonylphenyl ether (ethyleneoxide polymerization degree: 2 to 100), polyethylene glycol octylphenyl ether (ethyleneoxide polymerization degree: 4 to 50), and the like. A third group of compounds is an etherified compound of the first group of compounds and a higher fatty acid, for example, polyethylene glycol monolaurate (ethyleneoxide polymerization degree: 2 to 30), polyethylene glycol monostearate (ethyleneoxide polymerization degree: 2 to 50), polyethylene glycol monooleate (ethyleneoxide polymerization degree: 2 to 50), and the like.
Among the above-listed Component (C), an ester of an alcohol and a fatty acid and an ester of an alcohol and a dicarboxylic acid are preferably used in view of resistance to friction and wearing.
These esters of lubricants are contained in an amount of 0.1 to 10 parts by weight, preferably 0.2 to 5 parts by weight, based on the total amount of 100 parts by weight of (A) the polyoxymethylene resin and (B) the wollastonite. If the addition amount of the lubricants is less than 0.1 part by weight, the improvement in the sliding properties unfavorably becomes insufficient. If it is more than 10 parts by weight, the reduction of mechanical properties becomes unfavorably remarkable.
The polyolefin resin used for Component (D) of the present invention is a homopolymer, a copolymer of an unsaturated olefinic compound represented by the general formula (1) or a modified product thereof: 
wherein R1 represents a hydrogen atom or a methyl group; and R2 represents a hydrogen atom, an alkyl group or a carboxyl group, having a carbon number of 1 to 10, an alkylated carboxyl group having a carbon number of 2 to 5, an acyloxyl group having a carbon number of 2 to 5, or a vinyl group.
Specifically, a polyethylene (a high-density polyethylene, a medium-density polyethylene, a high-pressure low-density polyethylene, a linear low-density polyethylene and an ultra-low-density polyethylene), a polypropylene, an ethylene-propylene copolymer, an ethylene-butene copolymer, a polypropylene-butene copolymer, a polybutene, a hydrogenated product of a polybutadiene, an ethylene-acrylic ester copolymer, an ethylene-methacrylic ester copolymer, an ethylene-acrylic acid copolymer, an ethylene-vinyl acetate copolymer, and the like are exemplified. As modified products thereof, graft copolymers prepared by grafting one or more other vinyl compounds can be exemplified.
Of these, a polyethylene (a high-pressure low-density polyethylene, a linear low-density polyethylene and an ultra-low-density polyethylene), an ethylene-propylene copolymer, and an ethylene-butene copolymer are preferable. A weight-average molecular weight of these polyolefin type resins is not particularly restricted, but preferably 10,000 to 300,000, more preferably 10,000 to 100,000, further more preferably 15,000 to 80,000. When the weight-average molecular weight is lower than 10,000, the resistance to friction and wearing with the same material as of the present invention are caused to be unfavorably deteriorated. When it is higher than 300,000, the resistance to friction and wearing with the polyoxymethylene resin are caused to be unfavorably deteriorated.
The polyolefin polymer is added in an amount of 0.1 to 10 parts by weight, preferably 0.2 to 5 parts by weight, based on 100 parts by weight of a resin comprising (A) the polyoxymethylene resin and (B) the wollastonite. When the addition amount of the polyolefin polymer is 0.1 or less, the improvement of the sliding properties unfavorably becomes insufficient. When it is more than 10 parts by weight, the molded product is unfavorably peeled off.
For the polyoxymethylene resin composition of the present invention, there can be employed crystalline nucleating agents, antistatic agents, mold release agents, inorganic fillers other than wollastonite and pigments, which are not described in the present specification but have been conventionally and widely used for the polyoxymethylene resins.
The pigments include inorganic and organic ones. The inorganic pigments mean those commonly used for coloring resins, for example, they include zinc sulfide, zinc oxide, titanium oxide, barium sulfate, titan yellow, iron oxide, ultramarine, cobalt blue, a carbonate, a phosphate, an acetate, carbon black, acetylene black, lamp black, and the like. The organic pigments include pigments in the types of, for example, azo, isoindolin, disazo, monoazo, anthraquinone, heterocyclic, perinone, quinacridone, thioindigo, perylene, dioxazine, phthalocyanine, and the like.
Melt kneading methods of the polyoxymethylene resin composition of the present invention are not particularly limited. Generally, the resin composition is melt kneaded with a monoaxial extruder or a biaxial extruder. At this time, a processing temperature is preferably 180 to 240xc2x0 C. Further, the ways of feeding each component upon the extrusion are not particularly limited. For instance, the components may be fed according to the methods: (1) mixing the polyoxymethylene resin, a wollastonite, a lubricant and a polyolefin resin all at once and feeding the mixture from the main feeder of an extruder to melt knead; (2) feeding the polyoxymethylene resin and an additive from the main feeder of an extruder and then feeding a wollastonite to a molten resin from a side feeder provided at the middle of the extruder to melt knead; (3) feeding and kneading an additive agent after the polyoxymethylene resin and wollastonite are melt kneaded; or the like.